Wiki Carcinoma of the Brain

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A 65-yr old patient, with a history of lung cancer, is admitted to a healthcare facility with ataxia and syncope and a fractured arm as a result of falling. The patient undergoes a closed reduction of the fracture in the emergency department and undergoes a complete workup for metastatic carcinoma of the brain. The patient is found to have metastatic carcinoma of the lung to the brain and undergoes radiation therapy to the brain. Which of the following would be the principal diagnosis in this case?

The answer is "Metastatic carcinoma of the brain"

How come the lung cancer isn't stated in the answer? I just need to understand this question which will help me in solving tons of other codes similar to this one. Thanks!
The information is in the ICD-10 guidelines.

Encounter for treatment of secondary malignancy

When an encounter is for a primary malignancy with metastasis
and treatment is directed toward the metastatic (secondary)
site(s) only, the metastatic site(s) is designated as the
principal/first-listed diagnosis. The primary malignancy is
coded as an additional code.

If they were asking for the secondary DX then you would code the primary malignancy (lung neoplasm) or history of depending in the status.

Current malignancy versus personal history of malignancy

When a primary malignancy has been excised but further treatment,
such as an additional surgery for the malignancy, radiation therapy or
chemotherapy is directed to that site, the primary malignancy code
should be used until treatment is completed.

When a primary malignancy has been previously excised or eradicated
from its site, there is no further treatment (of the malignancy) directed
to that site, and there is no evidence of any existing primary
malignancy, a code from category Z85, Personal history of malignant
neoplasm, should be used to indicate the former site of the malignancy
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